1. Field of the Invention
The present invention relates to an exhaust gas purification catalyst.
2. Description of the Related Art
Recently, emission control on automobiles and the like has been strengthened. Therefore, in order to address this issue, various exhaust gas purification catalysts for purifying hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOX) and the like in exhaust gas have been developed.
Many exhaust gas purification catalysts contain palladium and/or platinum as catalytic metals. Palladium and/or platinum play a role for accelerating oxidation reactions of HC and CO and reduction reactions of NOX.
However, palladium and platinum have a problem that they are susceptible to poisoning by HC in exhaust gas. When palladium and/or platinum are poisoned by HC, NOX reduction reaction on these precious metals becomes difficult to occur. Therefore, a catalyst comprising palladium and/or platinum may sometimes show an insufficient NOX purification rate specifically under a rich atmosphere in which an HC concentration in exhaust gas is high.
Therefore, in order to suppress this, a technique comprising adding a salt of an alkaline-earth metal element to a catalyst comprising these precious metals has been suggested (see, for example, Patent Literature 1). When such constitution is adopted, poisoning of palladium and platinum by HC is suppressed by the action of the alkaline-earth metal element, whereby exhaust gas purification performances such as an NOX purification rate can be improved to some extent.
However, there is room for further improvement of exhaust gas purification performances of exhaust gas purification catalysts.